The present invention relates to optical transmission techniques, particularly single mode integrated optics for tele and data communication. Specifically, the invention relates to a wavelength selective device, to a switch and to a method thereby.
A number of methods for increasing the capacity of optical fibres in a network are known. One of these comprises utilizing wavelength division multiplexing (WDM) to increase the utilization of the bandwidth in the network, which, however, requires the provision of devices that are capable to multiplex, demultiplex and filter transmission channels that are transferred at different so-called optical carrier wavelengths in the network. To achieve selective routing of each individual wavelength channel, wavelength selective coupling devices, so-called wavelength selective switches, are required.
Several different principles for wavelength routing are known in the literature.
N. Takato et al describe in 128-Channel Polarization-Insensitive Frequency-Selection-Switch Using High-Silica Waveguides on Si, IEEE Photon. Tech. Lett., volume 2(6), pages 441-443, June 1990, a principle for a wavelength selective switch that relies on asymmetrical, cascade-coupled Mach-Zehnder interferometers (MZI). Hereby, a 1-N switch may be realized, wherein optical signals at N equidistant separated wavelengths may be coupled between N outputs, however, not completely individually to each other.
The publication A new type of tunable demultiplexer using a multileg Mach-Zehnder interferometer, by J.-P. Weber et al, Proc. ECIO ""97 EthE5, Stockholm, pages 272-275, 1997, shows an MMIMZI device (multimode interference Mach-Zehnder interferometer) that may be used for cyclic, wavelength selective routing.
T. Augustsson, Bragg grating assisted MMIMI coupler for wavelength selective switching, Electron. Lett., volume 34(25), pages 2416-2418, 1998 and WO 98/39686 describe an MMIMIBg device (Bragg grating assisted multimode interference Michelson interferometer) that offers complete channel-individual routing.
K. Okamoto et al discuss in 16 channel optical add/drop multiplexer using silica-based arrayed-waveguide gratings, Electron. Lett., volume 31(9), pages 723-724, 1995 an AWG-based configuration (AWG, Arrayed Waveguide Grating), with which individual routing in principle may be achieved.
Generally, the problems of the above-mentioned known techniques comprise large power losses, difficulties to keep the channel crosstalk at an acceptably low level and power variations between different channels caused by e.g. interference effects. The configurations which offer rather good performance are all related with high costs due to large and/or complicated structures.
Particularly the following limitations are true for the different above-mentioned techniques. The technique according to N. Takato exhibits pointed passbands, which may cause e.g. power losses. With the technique according to J.-P. Weber a very narrow area where the crosstalk performance is good (i.e. low crosstalk) is achieved. This is in principle possible to compensate for, but needs complex interference circuits to achieve a non-linear phase response in the Mach-Zehnder arms of the device. The technique according to T. Augustsson needs a large circuit if many channels are to be handled. Even if the theory behind the technique indicates low crosstalk, e.g. process-dependent scattering effects may increase the crosstalk. The device is probably particularly sensitive to scattering losses in respect of crosstalk, since the component is based on reflection in long Michelson interferometer arms. Finally, the technique described by K. Akamoto needs a large chip surface. It exhibits furthermore pointed passbands, which may be compensated for to the cost of high power losses.
It is an object of the present invention to provide a device for wavelength selective phase control of a wavelength channel comprised in a wavelength selective multiplexed optical signal.
It is a further object of the invention to provide one or several devices of the above mentioned type of high performance, preferably for implementation in a switch with completely individual routing possibilities as regards the wavelength channels comprised in a wavelength multiplexed optical signal.
It is hereby a particular object of the invention to provide one or several devices for realization of a switch in lack of one or several of the drawbacks that are associated with known art.
According to a first aspect of the present invention a device for wavelength selective phase control of a signal at a predetermined wavelength comprised in a wavelength multiplexed optical signal is provided, said device comprising an input port for inputting the wavelength multiplexed optical signal, a splitting means for splitting the input wavelength multiplexed optical signal into the signal at the predetermined wavelength and a signal comprising substantially the other wavelengths in the multiplex, a phase control means for phase-controlling one of the two splitted signals, a combining means for combining the splitted phase-controlled signal and the other splitted signal to achieve a wavelength multiplexed wavelength selectively phase-controlled optical signal and an output port for outputting the wavelength multiplexed wavelength selectively phase-controlled optical signal.
Preferably, the device also comprises a waveguide for the respective splitted signal, each connected between a respective output port of the splitting means and a respective input port of the combining means, whereby the phase control element is arranged at one of these waveguides. Preferably, the inventive device is arranged with substantially equal propagation distance for the splitted signals.
The splitting means and/or the combining means may be comprised of a Bragg grating assisted MMI structure (MMI, multimode interference) or a Bragg grating assisted MMIMZI structure (MMIMZI, multimode interference Mach-Zehnder interferometer).
The splitting means and the combining means may alternatively be composed of one and the same structure, whereby the device comprises a feedback means for the guidance of one of the two splitted signals from said structure through the phase control element for phase control and back to said structure.
In this case, said structure comprises preferably a coupler connected to two waveguides provided with Bragg gratings, where said coupler is arranged for inputting the wavelength multiplexed optical signal and outputting said wavelength multiplexed optical signal into the two waveguides provided with Bragg gratings, where the Bragg gratings are arranged for splitting the signal into the signal at the predetermined wavelength and the signal comprising substantially the rest of the wavelengths in the multiplex by reflection of one of the signals and transmission of the other signal, said feedback means being arranged for guiding the transmitted signal from said structure, through the phase control means for phase control and back to said structure, said Bragg grating being further arranged for combining the divided signals through transmission of the returned phase-controlled signal, and said coupler is further arranged for receiving the combined wavelength multiplexed wavelength selectively phase-controlled optical signal and for outputting the same at the output port of the device.
The feedback means may comprise a further coupler and a waveguide loop for guidance of the transmitted signal through the phase control means for phase control and back to the coupler, whereby the further coupler is arranged for inputting the signal transmitted through the Bragg gratings, for outputting the same in said waveguide loop, for connecting the phase-controlled signal guided through the phase control means, and for outputting the same in said waveguides provided with said Bragg gratings.
All the couplers are preferably composed of MMI waveguide structures, but they may alternatively be comprised of directional couplers.
Alternatively, the feedback means comprises part of the waveguides provided with Bragg gratings and a reflection means for reflection of the transmitted signal back in the waveguides provided with the Bragg gratings. In this respect the phase control means comprises two phase control elements localized in the respective prolongation of the waveguides provided with the Bragg gratings. The reflection means may here be constituted by a structure for total reflection or of a Bragg grating for reflection of the transmitted signal.
In alternative embodiments the splitting means and/or the combining means may comprise an optical circulator or a Y coupler.
The Bragg gratings in the splitting and combining means may either be arranged to reflect the signal at the predetermined wavelength or to transmit the signal at the predetermined wavelength.
The phase control means is preferably arranged for phase control of the signal at the predetermined wavelength.
The device can further comprise at least one phase modulation means for wavelength selective phase modulation of the wavelength multiplexed optical signal.
Furthermore, the present invention comprises a switch for switching of at least one wavelength channel comprised in a wavelength multiplexed multichannel optical signal, comprising at least one device of the above said kind and adjusted for said wavelength channel.
Preferably, the switch comprises an interference circuit, particularly a Mach-Zehnder interferometer structure.
It is a further object of the invention to provide at least one method for wavelength selective phase control of a signal at a predetermined wavelength comprised in a wavelength multiplexed optical signal.
According to a further aspect of the present invention a method is thus provided comprising receiving the wavelength multiplexed optical signal, splitting the received wavelength multiplexed optical signal into the signal at the predetermined wavelength and a signal comprising substantially the rest of the wavelengths in the multiplex, phase-controlling at least one of the two splitted signals, combining the splitted phase-controlled signal and the other divided signal to obtain a wavelength multiplexed wavelength selectively phase-controlled optical signal, and outputting the wavelength multiplexed wavelength selectively phase-controlled optical signal into a waveguide.
Further objects of the present invention will be apparent from the following specification.
An advantage of the present invention is that switches of very flexible functionality are obtained when they are being implemented using one or several inventive wavelength selective phase control devices.
Yet an advantage of the invention is that it provides for an excellent possibility to realize switches of improved performance in relation to known switches.
A further advantage of the inventive switch is that it in some respects exhibits more system tolerant properties in comparison with known technique.
Further advantages of the present invention will be apparent from the following specification.